Anesthesia apparatus commercially available commonly include respective flow control valves for controlling the flow or supply of oxygen and anesthesia gas(es), e.g., nitrous oxide, into a common manifold and from there to a patient breathing circuit. Most apparatus also include sensing means and indicating meters, e.g., flow or rotometers, to indicate the gas flow delivered through the flow control valves as well as other system conditions, e.g., gas pressure. The common manifold along with these other sensing means is referred to as the flowmeter assembly.
It is the responsibility of the operator of the anesthesia machine to guarantee that a minimum supply of oxygen is provided in the delivered gas flow. Notwithstanding this responsibility, various accidents have occurred over the past few years when the oxygen percentage decreased below a minimum safety level. Many such accidents have been a result of a failure in the oxygen supply, and inadvertent closing of the oxygen control valve or a misjudgment in the setting of the flows.
Various safety devices are known and commercially available and which respond to the pressure in the oxygen supply line. Such devices signal a decrease or total failure of the oxygen supply pressure. Such devices may also interrupt, or decrease, all gas flows other than oxygen in the event of a partial or total failure of oxygen supply pressure. However, prior art devices which function responsive to oxygen pressure have the major disadvantage that if the oxygen control valve is closed, such that no oxygen is delivered to the patient, the oxygen pressure will still exist in the supply line and the alarm device will not provide an alarm indication even though no oxygen is flowing.
In U.S. Pat. No. 4,191,952 (Schreiber), assigned to the same assignee as this application and whose disclosure is incorporated by reference herein, there is disclosed and claimed a low oxygen flow alarm system for anesthesia apparatus supplying oxygen through one pipeline into a manifold while supplying an anesthesia gas through a second pipeline into the manifold. The alarm system of that invention comprises a first pressure actuated means, e.g., an expandable/contractible chamber having a resilient diaphragm making up a wall of the chamber. The diaphragm is responsive to the oxygen pressure in the first pipeline and has a first output member, e.g., an end portion of a rod connected thereto so that the position of the rod is dependent upon the oxygen pressure. A second pressure actuated means, e.g., an expandable/contractible chamber having a resilient diaphragm making up a wall of the chamber, is also provided. The diaphragm of this chamber is responsive to the anesthesia gas pressure in the second pipeline and is connected to a second output member, e.g., the other end portion of the rod. Thus, the rod's position is dependent upon the anesthesia gas pressure. An alarm means, e.g., a lamp and/or annunciator and an associated switch, are coupled to the rod. The diaphragms act in opposition to each other on the rod. The switch of the alarm means is coupled to the rod to produce an alarm signal whenever the rod has been moved in the second direction to a predetermined position.
In U.S. Pat. No. 4,015,617 (Connolly) there is disclosed anesthesia apparatus providing a mixture of oxygen and nitrous oxide gas into a breathing circuit for the patient. The apparatus includes a flow control valve for adjusting the flow of oxygen into the breathing circuit and a nitrous oxide pressure regulator for regulating the nitrous oxide flow in response to monitored oxygen pressure. By varying the oxygen flow control valve, the flow of nitrous oxide is automatically varied to maintain a predetermined gas flow ratio.
While the device disclosed in the Connolly patent appears suitable for its intended purpose, it nevertheless suffers from at least one major drawback, namely, limited utility. In this regard, the Connolly system does not allow independent adjustment of nitrous oxide and oxygen flow. Thus, if one reduces the oxygen flow in the apparatus of the Connolly patent, the system will automatically make a corresponding reduction in the nitrous oxide flow.
In U.S. Pat. No. 4,328,823 (Schreiber), assigned to the same assignee as this application, and whose disclosure is incorporated by reference herein, there is disclosed and claimed an oxygen flow ratio controller for anesthesia apparatus supplying oxygen through one pipeline into a manifold while supplying an anesthesia gas through a second pipeline into the manifold. Moreover, this controller automatically regulates the ratio of oxygen gas to anesthesia gas provided into the patient breathing circuit while enabling independent control of oxygen and anesthesia gas so long as a threshold concentration of oxygen exists. This threshold concentration of oxygen is reflected by a critical oxygen gas to anesthesia gas ratio value, hereinafter known as the predetermined minimum threshold level. In particular, the controller comprises a flow control valve coupled to the second line for controlling the flow of anesthesia gas therethrough. A first expandable/contractible pressure chamber formed partly by a resilient flat diaphragm is provided, responsive to oxygen pressure in the first line. A second expandable/contractible pressure chamber formed partly by a resilient flat diaphragm is provided, responsive to anesthesia gas pressure in the second line. A common rod is connected to each of the diaphragms and to the flow valve. The diaphragms act in opposition to each other to move the rod and operate the flow control valve in response to the differential pressure monitored by the chambers.
The assignee of this application, N.A.D., Inc., has been selling a controller, referred to hereinafter as the ORMC (Oxygen Ratio Monitor Controller), which incorporates many of the features of U.S. Pat. No. 4,328,823. In addition, the ORMC operates as discussed below.
For high fresh gas (oxygen gas and anesthesia gas) flow rates (e.g., 2 liters/minute and greater) to the ORMC, the predetermined minimum threshold level remains constant (e.g., 25 percent). Under normal operating conditions, the ratio of oxygen gas to anesthesia gas is well above the predetermined minimum threshold level. Should the ratio of oxygen gas to anesthesia gas begin to fall toward the predetermined minimum threshold level, the flow control valve responds by restricting the flow of anesthesia gas, thereby establishing a new ratio that exceeds the predetermined minimum threshold level. In particular, under these conditions, the valve does not close. Instead, the valve is continuously being positioned to reflect the particular oxygen gas to anesthesia gas ratio, restricting the flow of anesthesia gas when the ratio approaches the predetermined minimum threshold level, and thereby correcting the flow of anesthesia gas to establish a new ratio above the predetermined minimum threshold level. However, in the unusual circumstance where the oxygen flow is accidentally cut off, the flow control valve would close.
For low fresh gas flow rates (e.g., less than 2 liters/minute), it is necessary to ensure that even greater concentrations of oxygen are being supplied to the patient. This is accomplished by providing for increased predetermined minimum threshold levels (well above 25 percent) for decreasing fresh gas flow rates. In other words, as the fresh gas flow rate decreases, the ORMC requires increasingly higher percentages of oxygen in the mixture than at the high flow gas rates or else the flow control valve begins to restrict the flow of anesthesia gas.
However, the ORMC requires a dynamic seal (formed by the rod passing through an O-ring ) between the flow control means and the second pressure chamber. This arrangement introduces friction and hysteresis into the system. In addition, should this dynamic seal ever fail, or if a leak should occur, the control valve could send uncontrolled amounts of anesthesia gas into the patient breathing circuit. Moreover, the adjustment of the ORMC predetermined minimum threshold level for low fresh gas flow rates is a "trial and error" method: an integral adjustment mechanism is adjusted either at the factory or by a field service technician. This must be followed by connecting the adjusted ORMC to a test assembly for determining if the correct adjustment was made. If it was not correctly adjusted, the method must be repeated. Furthermore, the large size of the ORMC requires that it be remotely located, requiring substantial external pneumatic lines connected between it and the anesthesia apparatus. Finally, the flat diaphragms used in the ORMC's first and second pressure chambers are subject to stretching and stressing which may cause the ORMC operation to drift, i.e., as the diaphragms stretch, the ratio of oxygen gas to anesthesia gas at which the flow control valve closes drifts away from the predetermined minimum threshold level, requiring adjustment by a field service technician.